Multimode monopulse horn antenna



Sept. 22, 1970 R. 1.. PIERROT MULTIMODE MONOPULSE HORN ANTENNA Filed July 8. 1968 Fig.2

tes

U.S. Cl. 343-786 2 Claims ABSTRACT OF THE DISCLOSURE In order to supply directly signals 2 and As+jAg or Ag+jAs, a multimode monopulse source comprises, between a radiating horn for TE TEGZ: T13 and TM modes and a magic T, a mode converter comprising two guide portions of rectangular section having respective ends opening into diametrally opposed sections of the horn.

The present invention relates to radiating sources for electromagnetic detection systems of the so called monopulse type in bearing and azimuth, i.e. sources in which the complete determination of a target position requires the formation of three signals (sum 2, difference in hearing As and difference in azimuth Ag). Consequently three corresponding channels whose complex gains must be strictly identical are necessary.

It is known that the reception circuits can be substantially simplified by forming the signal As-l-jAg, where j is the conventional operator for a phase-shift 1r/ 2: it sufiices to have two channels, one for the signal 2 and the other for the signal As-i-jAg. The formation of the two error voltages in azimuth and in bearing is obtained by means of amplitude-phase detectors supplied in quadrature.

A monopulse source of this type, supplying the three signals 23, As and Ag has been described in the French Pat. No. 1,512,406, issued on an application filed Dec. 21, 1966, for New Multimode Monopulse Source.

However such a source requires an additional coupler for forming the signal (As+jAg) which is fed to the difference channel.

It is an object of this invention to provide a multimode monopulse source supplying directly the signals 2 and As+jAg (or Ag+fAs which is the same with the exception of a constant phase-shift).

According to the invention there is provided a multimode monopulse source for directly supplying simultaneously signal 2 and signal As+jAg or Ag-HAs, where 2 is the sum signal and where As and Ag are respectively the azimuth and elevation difierence signals, said source comprising a radiating horn for TE TE TE; and 'IM modes to the exclusion of higher order modes, a four channel hybrid junction having a sum channel and a difierence channel decoupled from each other and two further channels coupled to both former channels and decoupled from each other and a mode converter inserted between said horn and said further channels, said mode converter comprising two guide portions of rectangular section having respective first ends opening into said horn, said ends being diametrally opposed to each other, and two further ends respectively coupled to said further channels of said junction.

For a better understanding of the invention and to show how the same may be carried into efiect, reference will be made to the drawings accompanying the following description and in which:

FIG. 1 is a perspective view of an assembly of a multimode monopulse source according to the invention;

atent FIGS. 2a, 2a, 2b, 2b and 2c, 20' are explanatory diagrams; and

FIG. 3 is an enlarged view of a portion of the source shown in FIG. 1.

It is known that in radar systems of the type considered the source, in so far as its dimensions and the operating frequencies are concerned, is so constructed that when the target is along the axis of the source, the picked up signal excites in the source the TE mode. When the azimuth angle is not equal to zero, modes TE and TE are excited and when the elevation angle is not zero, modes TE and TM are excited, in addition to the mode TE Of course if both azimuth and elevation angles are not zero all the four above mentioned modes are excited.

The monopulse source according to the invention shown in FIG. 1 comprises a horn C, formed in this embodiment by a flared mouth C and a rectangular guide C a magic T formed by a sum channel S, a difference channel D and two excitation arms T and T and a mode converter M which forms the transition between the magic T and the horn C. The mode converter M is coupled to the horn through two openings M and M which are diagonally opposite. As already mentioned, the horn C is dimensioned to permit excitation of the modes TE T 2, TE and TM of which the diagrams of FIGS. 2a, 2a", 2b, 2b, 2c, 20' show the amplitude distributions along the axes Ox and Oy.

The graphs 2a and 2a show, respectively, the amplitude distribution along Ox and 0y for the mode TE i.e. that corresponding to the excitation of the sum channel, since for this mode in horn C, the same mode is excited in the mode converter M. The graph 2b and 2b show the mode TE i.e. that corresponding to the difference channel for azimuth signals since this mode in the horn corresponds to azimuth signals in the mode TE in the difference channel. The graphs 2c and 2c correspond to modes TE and TM since these modes in the horn correspond to elevation signals in the mode TE in the difference channel.

The aforementioned correspondence result essentially from the fact that the magic T is coupled to the horn C through diagonally opposite openings M and M The horn C is energized, in a known manner, so that the waves in the mode TE while propagating therein, are phase-shifted by 1r/2 relative to those in the modes TEn and TM This phase-shift may be obtained either by achieving different propagation velocities by suitably selecting the dimensions of the horn, or by mounting a phase-shifter element, for example a pile along the axis of the guide. The first method has the advantage of not introducing a parasitic phase-shift of the waves in the mode TE and thus avoiding any additional phase compensation.

It will be recalled that the differential phase-shift go of the waves in the mode TE on one hand, and in the modes TE or TM on the other hand, over the length L, may be written as follows:

where ta and M are the wavelengths at the cut-01f 902 and Ag frequency corresponding to those modes and Written, respectively:

2ab w re It may be seen that by adjusting the height 12 without modifying the width a go may be made equal to an odd multiple of 1/2 without changing the propagation speed of the mode TE waves since for these the cut-off wavelength is independent of b (it is equal to 2a).

The phase-shift (p may also be adjusted by changing the length L of the guide. In all these cases the compensation of the parasitic diflerential phase-shift between the waves TE and TE will be effected by conventional kc =a and Ac For example, for a source for illuminating a circular paraboloid one may take b/a=0.8 and therefore kc =1.56b 1.25a. Since A must be equal to k1r/ 2 where k is an odd integer for \=32 mm. and a=45 mm. one finds, that L=66.66k; for k=1, L=66.6 mm.

The matching of the radiating horn for the modes T13 and TM must be made perfect to prevent multiple reflections between the mode converter which is not matched, and the horn, which might cause parasitic coupling between the odd modes.

Of course the invention is not restricted to the embodiment shown and described, which was given merely by way of example. Thus, the junction in which the signals of the channels S and D are formed from the signals supplied by the mode converter may be realized in any other way known in hyperfrequency techniques, such as, for example, triplate circuits, coaxial circuits, etc.

Needles to say, the antenna is both a receiver and an emitter antenna.

What is claimed, is:

1. A multimode monopulse source for directly supplying simultaneously signal 2 and signal As-l-jAg or Ag+jAs, where E is the sum signal and where As and Ag are respectively the azimuth and elevation difference signals, said source comprising a radiating horn for TE TE TE and TM modes to the exclusion of higher order modes, a four channel hybrid junction having a sum channel and a difference channel decoupled from each other and two further channels coupled to both former channels and decoupled from each other and a mode converter inserted between said horn and said further channels, said mode converter comprising two guide portions of rectangular section having respective first ends opening into said horn, said ends being diagonally opposed to each other, and two further ends respectively coupled to said further channels of said junction.

2. A multimode monopulse source according to claim 1, wherein said junction is a magic T.

References Cited UNITED STATES PATENTS 3,308,469 3/1967 Drabowitch 343-786 3,423,756 1/1969 Foldes 343786 ELI LIBERMAN, Primary Examiner U.S. Cl. X.R. 

